Ionic photoacid generators comprising an organic onium or organometallic cation and a non-nucleophilic counter anion have been shown to have utility as photochemically activated initiators for cationic addition polymerization in negative resists and polymer coating formulations or as similarly activatable latent acid catalysts for step-growth (or condensation) polymerization, depolymerization and the deprotection of functionalized polymers used in positive, chemically amplified photoresists. Common commercial ionic PAGs include onium and organometallic salts such as diaryliodonium and triarylsulfonium salts and (cyclopentadienyl)(arene)iron+ salts of the anions PF6−, SbF6−, CF3SO3−, C4F9SO3− and C8F17SO3−. In certain cases, these same salts may also photoinitiate free-radical addition polymerization and are useful in “dual cure” applications where a mixture of cationically sensitive and free-radically polymerizable monomers are polymerized either simultaneously or sequentially. Similarly, certain classes of these salts are known to be thermally-activatable curatives for cationic, step-growth and free-radical polymerizations.
A variety of neutral photoacid generators are also known in the art and these have found utility in photoresists and curing applications where a source of photogenerated latent acid is required to catalyze or initiate secondary chemical reactions. Among the more common neutral PAGs are the sulfonate esters where a covalently bonded organic radical, A, is directly attached to a incipient sulfonate anion, R—SO3-A, where R is generally an alkyl or aryl group, and may be highly fluorinated to impart high acidity to the latent sulfonic acid, R—SO3—H, produced by photo-induced cleavage of the O-A bond.
The migration of semiconductor manufacturing to ever smaller feature sizes is pushing the limits of optical lithography and increasing the need for new photoresist materials that can meet the demands of more advanced lithography platforms. A critical component of photoresist formulations is the photo-acid generator or PAG. PAGs are the photoactive ingredients in photoresists that produce acid upon irradiation. In a positive resist this photoacid generally serves to catalyze deprotection of the polymeric resist, thereby altering its solubility in a developer. In a negative resist the photoacid typically initiates cationic polymerization or curing of monomeric groups, resulting in crosslinking of the resin in the irradiated areas. In both cases this process is referred to as chemical amplification, since a single photon is responsible for catalyzing or initiating multiple chemical events. Most of the PAGs currently used in semiconductor microlithography are ionic in nature, comprising a photoactive cation and a negatively charged counterion.
Organic onium salts, especially those containing iodonium and sulfonium, cations, are particularly useful as PAGs in chemically amplified photoresist applications owing to their high quantum efficiency for acid production at commonly used exposure wavelengths. In positive photoresists used in semiconductor microlithography, a number of other features and functional properties have been identified as being critical to PAG performance. These include: 1) compositions that are free of metallic or semimetallic elements (i.e., dopant elements) that can alter the electronic properties of the semiconducting substrate (e.g., silicon), 2) high photoacid strength for maximum catalytic activity and photospeed, 3) low photoacid volatility to minimize acid contamination of imaging equipment and reduce imaging defects like T-topping , 4) low photoacid diffusivity for achieving high resolution and small feature sizes, and 5) thermal stability to prevent decomposition during prebake and postbake steps.
More recently, the toxicity, environmental persistence and bioaccumulation characteristics of PAG compositions has become an important consideration in determining their commercial viability. For ionic PAGs, all of these features and properties are determined or influenced by the chemical structure of the PAG anion. The structure of the anion directly determines the identity of the photo-acid produced upon irradiation of the PAG. Differences in the size, shape, and chemical makeup of the anion, X−, can lead to dramatic differences in the acidity, catalytic activity, volatility, diffusivity, solubility, and stability of the PAG or its' conjugate photo-acid, HX. These can in turn directly influence a variety of parameters related to photoresist performance, such as deblocking (or curing) efficiency, photospeed, post exposure bake (PEB) sensitivity, post-exposure delay stability, resolution, standing waves, image profiles, and acid loss (responsible for T-topping and the contamination/corrosion of exposure and processing equipment). There are currently very few PAG anions that provide both the requisite balance of properties as well as an acceptable EHS+R (environmental, health, safety and regulatory) profile for use in semiconductor photoresists. Consequently, the selection of ionic PAGs for semiconductor photoresist applications has become anion limited and there exists a pressing need within the industry for a greater selection of semiconductor-compatible PAG anions that offer desirable photoresist performance, along with safety and environmental sustainability.
Ionic PAGs have additional utility in the preparation of polymer coatings, sealants, encapsulants, and the like derived from cationically polymerizable monomers and oligomers. For many commercial applications, the polymerizable monomers are multifunctional (i.e., contain more than one polymerizable group per molecule), for example, epoxides, such as diglycidyl ethers of bisphenol A (DGEBA) and vinyl ethers, such as 1,4-cyclohexanedimethanol divinyl ether (CHVE). Mixtures of multifunctional monomers such as polyisocyanates and polyalcohols (polyols) or polyepoxides and polyalcohols can undergo acid-catalyzed polycondensation via a step-growth mechanism. Also included in this description are multireactive monomers—those that comprise two or more classes of reactive groups, such as, for instance, a monomer comprising both acrylate and isocyanate functionalites.
Compounds and materials comprising charged ions (i.e., salts) tend to have poor solubility in many organic solvents. As many useful types of compositions are based on organic systems, either organic polymer systems or organic monomer systems, reduced solubility in organic systems limits the field of utility of many ionic materials. Amongst the ionic materials that could benefit from increased solubility in organic systems are ionic PAGs (particularly those based on iodonium, sulfonium, diazonium, phosphonium and organometallic complex cations).
Synthetic modifications of the cationic portion of ionic PAGs have been made to improve their solubility in organic systems including unsymmetrical substitution of aromatic groups in aryl iodonium and sulfonium salts with different alkyl substituents. Alternatively, the use of reactive diluents or solid dispersants has also been disclosed.
The nature of the counteranion in an ionic PAG can influence the rate and extent of cationic addition polymerization. For example, J. V. Crivello, and R. Narayan, Chem. Mater., 4, 692, (1992), report that the order of reactivity among commonly used nonnucleophilic anions is SbF6−>AsF6−>PF6−>BF4−. The influence of the anion on reactivity has been ascribed to three principle factors: (1) the acidity of the protonic or Lewis acid generated, (2) the degree of ion-pair separation in the propagating cationic chain and (3) the susceptibility of the anions to fluoride abstraction and consequent chain termination.